In the related art, an oil separator that performs a gas-liquid separation of oil from blow-by gas is known (for example, refer to JP2012-255372A (Reference 1)).
In above Reference 1, the oil separator that separates the oil included in the blow-by gas of an internal combustion engine is disclosed. The oil separator described in Reference 1 is disposed inside a cylinder head cover. The oil separator has a partition wall which is formed for a plurality of small holes to be penetrated in a flow path of the blow-by gas, and a collision plate disposed in a downstream side of the partition wall in an inside. Thereby, the blow-by gas of which a flow rate is increased when passing through the plurality of the small holes performs an inertial collision against the collision plate by using an inertial motion, and thus oil mist is separated from the blow-by gas. Liquid oil that is grown into droplets by adhering to a front surface of the collision plate is configured to be discharged via a drain pipe (discharge passage) disposed on a bottom surface of the oil separator.
However, in an oil separator described in above Reference 1, it is possible to separate the oil mist only by an inertial collision of blow-by gas against a collision plate. That is, while the oil mist of which a particle diameter is large to some extent can be collected by the inertial collision using an inertial motion, since mass of respective particles are too small, the oil mist of which an average particle diameter is reduced in diameter (micronized) up to approximately 1 μm or less is taken away to an outside of the oil separator along with blow-by gas flow, without performing the inertial collision against the collision plate. For this reason, there is a problem that the oil mist reduced in diameter cannot be effectively separated from the blow-by gas. Specially, in an internal combustion engine with a supercharger or the internal combustion engine in which a combustion state of an air-fuel mixture is set at a high compression ratio, since the oil mist included in the blow-by gas is likely to be reduced in diameter, separation capacity (collection performance) of the oil mist reduced in diameter is desired to be improved.